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Science 2

This document discusses the benefits of a finer grain size in castings, including improved resistance to hot tearing during solidification, improved weldability and inspectability, higher strength and ductility, and reduced porosity. However, it notes that not all alloy systems benefit from grain refinement and some require large grains for high temperature creep resistance. It also explains that while classical metallurgy explains some benefits of fine grains, the influence of bifilm defects at grain boundaries can sometimes override conventional considerations and must be accounted for.

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Nest Necture
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105 views1 page

Science 2

This document discusses the benefits of a finer grain size in castings, including improved resistance to hot tearing during solidification, improved weldability and inspectability, higher strength and ductility, and reduced porosity. However, it notes that not all alloy systems benefit from grain refinement and some require large grains for high temperature creep resistance. It also explains that while classical metallurgy explains some benefits of fine grains, the influence of bifilm defects at grain boundaries can sometimes override conventional considerations and must be accounted for.

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Nest Necture
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© Attribution Non-Commercial (BY-NC)
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Chapter 9 Structure, defects and properties of the finished casting

9.1 Grain size 9.1.1 General


The development of small grains during the solidification of the casting is generally an advantage. When the grain size is small, the area of grain boundary is large, leading to a lower concentration of impurities in the boundaries. The practical consequences that generally follow from a finer grain size are: 1. Improved resistance to hot tearing during solidification. 2. Improved resistance to cracking when welding or when removing feeders by flame cutting (for steel castings). 3. Reduced scattering of ultrasonic waves and X-rays, allowing better non-destructive inspection. 4. Improved resistance to grain boundary corrosion. 5. Higher yield strength (because of Hall-Petch relationship). 6. Higher ductility and toughness. 7. Improved fatigue resistance (including thermal fatigue resistance). 8. Reduced porosity and reduced size of pores. This effect has been shown by computer simulation by Conley et al. (1999). The effect is the consequence of the improved intergranular feeding and better distributed gas emerging from solution. Improved mass feeding will also help as described in section 7.4.2. 9. Improved hot workability of material cast as ingots.

However, it would not be wise to assume that all these benefits are true for all alloy systems. Some types of alloys are especially resistant to attempts to reduce their grain size, while others show impaired properties after grain refinement. Furthermore, this impressive list is perhaps not so impressive when the effects are quantified to assess their real importance. These apparent inconsistencies will be explained as we go. In addition, important exceptions include the desirability of large grains in castings that require creep resistance at high temperature. Applications include, in particular, ferritic stainless steel for furnace furniture and high-temperature nickel-based alloy castings. Single-crystal turbine blades are, of course, an ultimate development of this concept. These applications, although important, are the exception, however. Because of the limitations of space, this section neglects those specialized applications that require large grains or single crystals, and is devoted to the more usual pursuit of fine grains. Some of these benefits are explained satisfactorily by classical physical metallurgy. However, it is vital to take account of the presence of bifilms. These will be concentrated in the grain boundaries. The influence of bifilm defects is, on occasions, so important as to over-ride the conventional metallurgical considerations. For instance, in the case of the propagation of ultrasonic waves through aluminium alloy castings, this was long thought to be impossible. Aluminium alloys were declared to be too difficult. They were thought to prevent ultrasonic inspection because of scatter of the waves from large as-cast grains.

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